I met Hamous in sector F12, but something is wrong. I recruited him, but instead of truck there is a guy my merc can... get inside, so to speak.

I select a merc, hover cursor over that guy, cursor changes to steering wheel icon. My merc gets near the guy and disappears with car door shutting sound. Only one merc can get into this human-truck...

That version is very old although I don't know if there was a bug with Hamous. You should at least install the latest official update 7609 from the download section of the wiki.

If you don't plan to use mods anyway you can as well install Depri's latest SCI which is the development version of JA2 1.13. It has all the new features and bug fixes (and of course some new bugs ;-) ).

If you want to use mods you need to check the mod documentation for the game version that it requires.